1. Field of the Invention
The invention relates to stitch errors in printing and, more particularly, to techniques for obscuring stitch errors in printing as perceived by the eye of a viewer.
2. Description of the Prior Art
Fluid ejecting devices such as, for example, ink jet printers, fire drops of fluid from rows of nozzles of an ejection head. The nozzles are usually fired sequentially in groups beginning at one end of the head and continuing to the other end of the head. While the nozzles are being fired, the head moves at a rate designed to advance it by a resolution distance before the next firing sequence begins. If the nozzles are not fired simultaneously, the rows of nozzle are usually tilted so that drops fired from all nozzles land in a substantially vertical column. The ejection head can have one or more dies, each die having a plurality of nozzles. Some devices have ejection heads with only one die, and some devices have ejection heads with multiple dies. If an ejection head has multiple dies, the dies can be, for example, arranged vertically with respect to one another so that the head can eject more drops in a single swath of the head compared to a head having a single die.
The line at which the swaths ejected by adjacent dies, or at which the adjacent swaths, meet is called the stitch joint. Stitch joint error exists when the swaths meeting at the stitch joint meet in such a way that the resulting arrangement of drops at the stitch joint of a printed image is undesirable. Because the spacing of the stitch joint errors is typically xc2xd to 1 times the printing width of the print head (typically xc2xcxe2x80x3 to xc2xdxe2x80x3), the stitch joint errors are very noticeable because the human eye is very sensitive to this spatial frequency region.
Stitch joint error can be, for example, the result of a gap between the drop of one die adjacent the stitch joint and the drop of an adjoining die adjacent the stitch joint. Such a gap can be the result of the same firing sequence being used for the nozzles of both dies. A similar stitch joint error can be caused when the same nozzle firing sequence is used for each swath of a single die ejection head.
An earlier attempt to reduce stitch joint error is disclosed in commonly assigned U.S. Pat. No. 6,338,544 to Drake et al. The focus of the patent is that stitch joint error can be reduced by firing the nozzles of adjacent dies in a multi-die ejection head using different firing sequences. Similarly, the nozzles of a single die ejection head can be fired using different sequences in adjacent swaths of the ejection head. By firing the nozzles in different sequences as discussed above, the drops at the stitch joint can be positioned closer to each other than they would be if the same firing sequence was used for each die/swath. By reducing the distance between the drops on either side of the stitch joint, the location of the stitch joint becomes less apparent.
It was with knowledge of the foregoing state of the technology that the present invention has been conceived.
In a technique for obscuring the stitch error perceived by the eye of a viewer, a first plurality of drops of fluid are ejected in a first firing sequence at a medium in a first swath from a print head including at least one die having a plurality of nozzles while moving in a first direction relative to the medium. Thereafter, the medium is advanced in a second direction substantially perpendicular to the first direction. The print head is again moved in the first direction and a second plurality of drops of the fluid are fired at the medium in a second swath adjacent the first swath. A controller randomly modulates the spot size resulting from the drops of the fluid. The stitch error may be a misplacement of the second swath relative to the first swath or a nonuniformity in drop size and at least the second plurality of drops includes a random variety of drop sizes.
This disclosure document describes the use of spot size modulation to introduce noise in the stitch area of two overlapping print die. The noise can be useful in minimizing the perception of defects related to the stitching of the two arrays. These defects can result from non-ideal alignment of the two die, or can result from a difference in optical density (spot size) of the two die. The spot modulation can be achieved by a strategy of multiple drop overlap as in some known thermal ink jet products, or can be achieved by actual drop size modulation as in known piezoelectric ink jet print heads.
One known way to increase the productivity of a single pass mode in a thermal ink jet printer is to use multiple print heads, e.g. increasing the black printing productivity by using two black print heads. However, the precision mechanical alignment of two print heads is difficult and expensive. In addition, if the alignment is less than perfect, a systematic error results that is repeated at the spatial frequency of the print head/so is very visible. An alternate method is to coarsely align the two black print heads and use a checkerboarding scheme where each print head prints part of the pixels, to introduce spatial noise into the overlapped arrays to minimize the perception of the systematic alignment error.
This proposal suggests that another way of minimizing the perception of a stitch defect in the overlap region is by drop modulation. Random drop modulation can introduce noise into the overlap region which will break up the systematic nature of the defect. In addition, if the misregistered stitch is visible as a lighter band, modulating for bigger spots will tend to obscure this defect. In fact, even in butted array strategies (i.e., where array overlapping is not possible) spot modulation can obscure the lighter or darker stitch bands by a complementary compensation. Compensation could be done at the factory or by user selection.
Spot Modulation for other than the purpose of the present invention is commercially achieved by known piezoelectric printers and is also achieved in some print heads by means of prepulse control. It is likely that both piezoelectric and thermal ink jet print head technology will advance the capability of drop size modulation so this technique to introduce noise to obscure systematic defects will become more and more capable.
A primary feature, then, of the present invention is the provision of techniques for obscuring stitch errors in printing as perceived by the eye of a viewer.
Another feature of the present invention is the provision of such a technique which hides stitch errors in overlapping or butted print dies.
Yet another feature of the present invention is the provision of such a technique which uses spot size modulation in the print head, noise being created by random drop modulation in the stitch region which would be used to hide structure from such a stitch region.
Still another feature of the present invention is the provision of such a technique according to which noise is biased toward larger spot sizes for darkening or toward smaller spot sizes for lightening, such that the density matches the desired density in the non stitch region.
Other and further features, advantages, and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of this invention, illustrate one of the embodiments of the invention, and together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.